Grid size effect on isodose distribution of volumetric modulated arc therapy (VMAT) and intensity modulated radiotherapy (IMRT) plans of stage III lung cancer patients which calculated with AAA and Acuros XB algorithms

Dose calculation algorithms are decisive to apply prescribed dose precisely in radiotherapy practice. In this study, the effect of grid size on organ at risk (OAR) and target doses were studied, in patients with stage III, inoperable non-small cell lung cancer (NSCLC) receiving radiotherapy, used VMAT or IMRT radiotherapy techniques while Acuros XB or AAA algorithms for dose calculation. VMAT and IMRT treatment plans were created using Acuros XB and AAA dose calculation algorithms in different grid sizes for selected 10 patients. Target and OAR doses, lung V20 and hearth V40 volumes according to grid sizes were analyzed using Dose Volume Histograms (DVH). Grid size showed significant effects on PTVtm covarege and high dose region (p=0,000 - 0,045); also showed significant effects OAR doses, heart V40 and lungs V20 volumes (p=0,005 – 0,037) using the same technique and algorithms. Using same grid sizes and same algorithms and different techniques resulted significant differences in high dose region of PTVtm (p=0,001 – 0,0096); whereas no significant difference in OAR doses, lung V20 and heart V40 volumes (p=0,959 – 0,139). When same grid size and same technique but different algorithms used in the plan, difference in coverage of PTVtm (p=0,000); doses to OAR's and lungs V20 heart V40 volumes were significant (p=0,005 – 0,047). Consequently, for the accuracy of dose calculation in low density area and hot point doses to serial organs which has in vital importance selection of the slice thicknesses of computed tomography (BT) images and grid size resolutions are important.